The present invention relates to a lead frame for a plastic encapsulated semiconductor device which can be used with relatively high power.
Plastic encapsulation of semiconductor devices has been recently developed. Manufacture of relatively high-power transistors includes plastic encapsulation. In these devices, substrate supports of a lead frame to which semiconductor substrates or elements are mounted also serve as heat sinks. Rear surfaces of the substrate supports carry no semiconductor elements and are not covered with plastic but are open to the atmosphere. With the above structure, when mounting the plastic encapsulated power semiconductor devices on various types of electrical apparatus, the exposed sides of the substrate supports are thermally coupled to radiators. In this case, insulating plates or films must be interposed between the substrate supports and the radiators, which must be electrically insulated from each other. For this reason, the mounting and assembly operations become cumbersome and time-consuming.
In order to eliminate the above drawbacks, the rear surface of each substrate support is covered with a thin resin layer to provide insulation of the substrate supports from the external radiators. In the lead frame used for this purpose, one external lead connected to a first connecting band extends from one edge of each substrate support, and two strips connected to a second connecting band extend from the other edge of each substrate support. For plastic encapsulating inside molds, the external lead and the two strips of each substrate support are firmly clamped between the upper and lower molds. Thus, the insulating thin resin layer is uniformly formed on the rear surface of each substrate support.
However, in this case, the lead frame may be thermally deformed by the heat of the plastic encapsulation process. Further, since a space between the rear surface of each substrate support and the lower mold is very narrow, the plastic does not flow well, resulting in defective molding.